Many processes, such as pig iron and steel production, generate byproducts that are rich in iron oxide, but are in the form of fine particles or sludge. Many approaches have been proposed for converting the iron oxide byproduct into commercially viable metallic iron which can be subsequently melted and refined into a metal product. Typically, the iron oxide containing material is combined with a binding agent, and the components are pelletized or otherwise agglomerated and subjected to high temperatures in the presence of a reducing agent. In the final step the iron oxide agglomerate is reduced to metallic iron. Agglomeration of the particles is necessary prior to the reduction step because the reduction gas velocities would blow finely divided material out of the reaction device.
U.S. Pat. No. 4,063,930 to Kusner et al. discloses a process in which particulate iron oxide dust is ground with lime and compacted at temperatures of about 1800° F. The compacted pellets are then subjected to heating in a reducing environment to convert the iron oxide to a ferrous state.
U.S. Pat. No. 3,895,088 to Goksel describes a method for producing iron oxide agglomerations for recovery of iron-rich byproducts of steel factories. The Goskel method entails blending together steel dust, calcium/magnesium oxide, a siliceous material, and optionally a carbonaceous material. The mixture is then moistened with water and pelletized. The pellets are then subjected to hydro-thermal conditions in a steam autoclave to provide integral, high strength agglomerates. A similar process is disclosed in U.S. Pat. No. 4,528,029 to Goksel which is directed to the formation of iron-oxide agglomerates with pyrolyzed carbonaceous materials.
U.S. Pat. No. 5,554,207 to Bogdan et al. teaches a method for recycling waste particulate iron oxide, where the iron oxide particles are agglomerated using water-insoluble thermoplastic resins as binding agents.
U.S. Pat. Nos. 5,865,875 and 6,270,551 both to Rinker et al. describe a process where an iron oxide material and carbonaceous material are agglomerated under high temperatures, without the presence of a binding agent to form “green compacts.” The green compacts are then added to a rotary hearth furnace to reduce the iron oxide.
U.S. Pat. No. 6,579,505 and U.S. Pat. No. 6,811,759, both to Tsuchiya et al., relate to a method of producing iron oxide pellets with improved strength by combining the iron oxide component with a carbonaceous material, an inorganic coagulant such as bentonite, and an organic binder such as starch. The materials are combined with water and pelletized into green compacts and subsequently dried prior to the reduction step.
A rotating hearth furnace is used to reduce iron oxide in numerous processes described in patents assigned to Kobe Steel. For example, U.S. Pat. No. 6,254,665 to Matsushita et al. relates to a method of producing reduced iron agglomerates by heating a composition of iron oxide and a carbonaceous substance in a moving hearth furnace. U.S. Pat. No. 6,152,983 to Kamijo et al. describes the reduction of iron oxide containing pellets in a rotary hearth furnace, where the pellets further include zinc oxide and a carbonaceous material. According to Kamijo et al., the pellets are heated to reduce the zinc oxide to zinc, to evaporate the zinc off of the pellets, and to reduce the iron oxide to iron.
Additional references of interest include U.S. Pat. No. 6,258,149 to Sugiyama et al.; U.S. Pat. No. 6,592,647 to Hino et al.; U.S. Pat. No. 6,605,130 to Takenaka et al.; and U.S. Pat. No. 6,918,947 to Maki et al.
Despite the advancements in iron waste reclamation, many conventional processes do not produce iron agglomerates with sufficient strength. For example, many of the above processes use expensive rotary hearth furnaces because the agglomerates lack the strength to withstand the agitation associated with more economical alternatives such as rotary kilns. Indeed, the prior art teaches that the pellets need to be subjected to complicated autoclaving processes in order to achieve suitable strength. Furthermore, the agglomeration and reduction processes in much of the prior art relates to the production of pyrophoric metallic iron, which must be further processed (e.g., by briquetting) to render safe for shipping and handling.
It has been surprisingly discovered according to the present invention that iron containing product may be conveniently and economically produced in a non-pyrophoric form, without the need of additional briquetting or processing steps. According to one aspect of the invention, iron oxide is agglomerated with a carbon source, zinc oxide and calcium or magnesium oxides. In addition to being non-pyrophoric when processed properly, the inventive combination enables the formation of the agglomerates (prior to reduction) at low temperatures and low pressures, yet which still exhibit high strength. Other desirable features of the invention include (1) the production of non-pyrophoric metallic iron; (2) low equipment costs; (3) low operating costs; (4) fewer and simpler process steps; (5) safe handling; (6) shipping convenience and (7) high energy efficiency.